Diagram showing an initial velocity vector for three vehicles and the corresponding angular displacement from the initial wheel positions required to change the direction of the initial velocity vector by the same value when turning using various three-wheeled car steering mechanism configurations

A three-wheeler is a vehicle with three wheels, either "human or people-powered vehicles" (HPV or PPV or velomobiles) or motorized vehicles in the form of a motorcycle, all-terrain vehicle (ATV) or automobile. Other names for three-wheelers include trikes, tricars and cyclecars. The term tricycle is used somewhat interchangeably, but the term three-wheeler is more often applied to motor vehicles. They can be legally classed as either automobiles or motorcycles.

Many three-wheelers which exist in the form of motorcycle-based machines are often called trikes and often have the front single wheel and mechanics similar to that of a motorcycle and the rear axle similar to that of a car. Often such vehicles are owner-constructed using a portion of a rear-engine, rear-drive Volkswagen Beetle in combination with a motorcycle front end. Other trikes include ATVs that are specially constructed for off road use. Three-wheeled automobiles can have either one wheel at the back and two at the front, (for example: Morgan Motor Company) or one wheel at the front and two at the back (such as the Reliant Robin).

Due to its superior safety when braking, an increasingly popular form is the front-steering "tadpole" or "reverse trike" sometimes with front drive but usually with rear drive. A variant on the 'one at the front' layout was the Scott Sociable, which resembled a four-wheeler with a front wheel missing.[1]

Three-wheeler cars, usually microcars, are often built for economic reasons: in the UK for tax advantages, or in the US to take advantage of lower safety regulations, being classed as motorcycles. As a result of their light construction and potential better streamlining, three-wheeled cars are usually very economical to run.

Three-wheeler transport vehicles known as Auto Rickshaws are a common means of public transportation in many countries in the world. Auto rickshaws are an essential form of urban transport in many developing countries such as India, and a form of novelty transport in many Eastern countries.

Trihawk, a Tadpole-type trike manufactured in California during the 1980s

A configuration of two wheels in the front and one wheel at the back presents two advantages : it has improved aerodynamics, and that it readily enables small lightweight motorcycle powerplant and rear wheel to be used. This approach was used by Messerschmitt kr200 and by the BMW Isetta. Alternatively, a more conventional front-engine, front wheel drive layout as is common in four-wheeled cars can be used, with subsequent advantages for transversal stability (further front location of CG) and traction (two driven wheels instead of one).

For the lowest wind resistance (which increases fuel efficiency), a teardrop shape is desirable.[citation needed] A teardrop is wide and round at the front, tapering at the back. The three-wheel configuration allows the two front wheels to create the wide round surface of the vehicle. The single rear wheel allows the vehicle to taper at the back. This approach is used by the Aptera 2 Series. This idea has also been used by Elio Motors to develop a new low cost, high fuel mileage vehicle to be released in 2015. It is also used by Myers Motors for both its single passenger NmG and upcoming 2-passenger Duo.

To improve stability some three-wheelers are designed as tilting three-wheelers so that they lean while cornering like a motorcyclist would do. The tilt may be controlled manually, mechanically or by computer.

A tilting three-wheeler's body and or wheels tilt in the direction of the turn. Such vehicles can corner safely even with a narrow track.

Several configurations are practical. Two front wheels and one rear, where all three wheels tilt, use the acronym 2F3T (i.e. two front three tilt). Other variations 1F1T, where only the front wheel tilts, an example being the Vandenbrink Carver, made in the Netherlands.

Tilting can be actively controlled by applying force between the paired wheels. A "free leaner" is balanced by steering, like a single track vehicle. This still leaves some advantages in traction, and the tilt can be locked for parking faster than a stand can be deployed.

Any extra laterally spaced wheel can benefit the vehicle by providing stability gains if there is a mechanism to lock the laterally spaced wheels at any given moment relative to the road plane. In some countersteered vehicles this benefit is generally only available when stopped.

In other types where the tilt angle is directly controlled this additional stability is constant at all speeds.

In a countersteered vehicle a loss of traction will result in a fall whereas in a tilt controlled design a fall may not occur.

Various electric and hydraulic systems have been employed to modify TTW behaviour, with notable success on the Carver.

Due to the tilting, there is not necessarily any load transfer between the wheels in cornering, so the rule about tadpoles (2F or "reverse trike") understeering and Deltas (1F or standard trike) oversteering does not necessarily apply

In the U.S the National Highway Traffic Safety Administration defines and regulates three-wheeled vehicles as motorcycles.[5]Licensing requirements vary on a state-by-state basis. In other jurisdictions, such as British Columbia, Canada, and Connecticut, a three-wheeled vehicle with an enclosed passenger compartment or partially enclosed seat is considered an automobile.

When the single wheel is in the front (the "delta" form, as in a child's pedal tricycle), the vehicle is inherently unstable in a braking turn, as the combined tipping forces at the center of gravity from turning and braking can rapidly extend beyond the triangle formed by the contact patches of the wheels. This type, if not tipped, also has a greater tendency to spin out ("swap ends") when handled roughly.

With two wheels in the front (the "tadpole" form or "reverse trike") the vehicle is far more stable in braking turns, but remains more prone to overturning in normal turns compared to an equivalent four-wheeled vehicle, unless the center of mass is lower and/or further forward. Motorcycle-derived designs suffer from most of the weight being towards the rear of the vehicle.

Due to the incidence of injuries related to their use, a 10-year ban, entirely voluntary for manufacturers, was placed on the sale of new three-wheeled ATVs in the United States in January 1988.[citation needed] However, it should be noted that more injuries were sustained by riders by not applying a proper riding technique, and lack of wearing proper safety gear such as helmets and riding boots. In a search conducted by the Consumer Product Safety Commission, it was determined that "no inherent flaw was found in the three wheel design".

In the British TV series Mr. Bean, a Reliant Regal three-wheeler was often driving around town. The car was sort of a nemesis to Mr. Bean, as he would drive in such a manner as to cause the car to roll over and even refused a ride from the three-wheeler when hitchhiking.[6]

eTuk USA have taken the tuk tuk, which is omnipresent in Asian cities, and re-designed it for the US Market, including an all-electric motor. visit their website (www.etukusa.com) to see the vehicle line-up ranging from 3/6 passenger vehicles and small food/vending trucks.